Question

how does the circulatory system work at the tissue level
nutrients are delivered throughout the body.
cardiac muscle contracts.
the heart pumps the blood.
blood cells carry nutrients.

Explanation:

The question asks about the circulatory system at the tissue level. Cardiac muscle is a type of tissue. When cardiac muscle (a tissue - level component) contracts, it is a tissue - level function related to the circulatory system. Nutrient delivery throughout the body is more of an organ - system level (involving blood vessels, heart etc. as a whole). The heart pumping blood is an organ - level function (heart is an organ). Blood cells carrying nutrients is a cellular - level function.

Answer:

Cardiac muscle contracts.